In wireless communication systems, such as third generation partnership project (3GPP) long term evolution (LTE) systems, downlink and uplink transmissions may be organized into two duplex modes: frequency division duplex (FDD) mode and time division duplex (TDD) mode. The FDD mode uses a paired spectrum where the frequency domain is used to separate the uplink (UL) and the downlink (DL) transmissions. The TDD mode, on the other hand, uses a common spectrum and relies on the time multiplexing to separate the UL and the DL transmissions.
In wireless communication systems, such as LTE systems, a User Equipment (UE) may transmit Hybrid Automatic Repeat reQuest (HARQ) bits in response to Physical Downlink Shared Channel (PDSCH) transmissions received on the DL. A HARQ bit may be an acknowledge (ACK), which signals successful receipt of data. A HARQ bit may also be a negative acknowledgement (NACK), which signals that data was not received successfully. A UE may transmit HARQ bits on a Physical Uplink Control Channel (PUCCH).
In LTE-advanced (LTE-A) systems, carrier aggregation (CA) enables multiple component carriers (CCs), which use up to 20 MHz bandwidth each, to be aggregated to form a wider total bandwidth. To support CA, a LTE-A UE may simultaneously receive or transmit on one or multiple CCs. These multiple CCs may be from the same evolved NodeB (eNB) or from difference eNBs. In a FDD system, the number of CCs aggregated in the DL may be different from the CCs in the UL and the bandwidths configured for the UL may also be different from the bandwidths for the DL.
For CA, a UE may have one independent HARQ entity per CC and in each of uplink or downlink. Each UE may have one Radio Resource Control (RRC) connection with the network. The serving cell handling RRC connection establishment/re-establishment/handover is referred to as the Primary Cell (PCell). The carrier corresponding to the PCell is termed the primary component carrier (PCC). Other serving cells are referred to as secondary cells (SCells) and their corresponding carriers are referred to as secondary component carriers (SCC). The carriers could be aggregated intra-band and/or inter-band.